Expose SchemaStore as a referencing.Registry.

[Julian]

This makes e.g.:

    import jsonschema
    import schemastore
    jsonschema.validate(
        instance={},
        schema={
            "$ref": "https://json.schemastore.org/github-action.json",
        },
        registry=schemastore.registry(),
    )

work with no additional twiddling.

@ssbarnea I mentioned briefly wanting to do something with this package ages ago but completely got tied up with other things. How involved do you want to be?

I think you added me to this repo, are you OK with me simply refactoring things even beyond this PR, or do you want to look at them? Are you willing to add me to PyPI as well? And/or are you OK with moving this to the python-jsonschema org?

[Julian]

Going to assume you're good with anything here! Will be making a few more changes, and doing the transfer -- if you have any ideas/preferences always feel free to mention!